Electrostatic powder coating apparatus requires dry powder to be conveyed to a spray head in fluidized form, electrostatically charged and dispensed from the spray head towards the article. In known coating apparatus (powder spray guns) powder is fluidized in a container and conveyed to the spray head (gun) using compressed air. It is dispensed from the spray head by using a deflector, diffuser or other spray pattern shaper. The powder is charged electrostatically by either corona charging, using high tension pointed electrodes, or friction charging, alone or in combinations. The powder particles are carried to the articles under electrostatic field attraction, pneumatic force and their own momentum. The fine powder spray from the spray head, in a defined fan pattern in the plane of the article, coats the article surface. There are various limitations, like the Faraday cage effect from corona charging, chemical formulation of the powder for friction charging.
The powder application process can be manual or automatic, using mechanical means like reciprocators, depending on the size and number of articles to be coated in a stipulated period.
An alternative to pneumatic powder spraying uses an electrostatic fluidized bed, wherein articles are introduced into a fluidized powder bed, and coating is carried out under electrostatic field only (without the use of air to carry powder to the articles). This process can also be in manual or automated versions.
Both the known processes, spray gun using pneumatic pressure, and fluidized bed using pure electrostatic attraction, have their advantages and limitations. However, the pneumatic type is more versatile and is used far more than is the fluidized bed.
Pneumatic applicators normally have an optimum value of powder throughput of 200 gms/min. though its maximum output can be 500 gms/min. with low charging and transfer efficiency. To achieve coating on opposite sides, a minimum of two guns are required spraying from opposite ends. For jobs with four sides, sometimes four applicators are used with indexing. The automatic applicators are vertically reciprocated to cover the total height of the articles. The number of applicators to be used can be large in order to match the speed of article, travel time, and its height or configuration, as articles are facing applicators for very small amount of time. For small components pneumatic force dispensing of powder can cause strong oscillations as the articles are often of light weight.
In the case of fluidized bed coating, the Faraday cage effect is a major limitation for articles of complex shape. A further disadvantage is that the thickness of the coating, and therefore of the final film, varies to an unacceptable extent on articles above about 50 mm in height.